1. Field of the Invention
The present invention relates to electronic semiconductor devices, and, more particularly, to quantum well devices in which carrier resonant tunneling through the well is modulated.
2. Description of the Related Art
Quantum well devices are known in various forms, heterostructure lasers being a good example. Quantum well heterostructure lasers rely on the discrete energy levels in the quantum wells to achieve high efficiency and typically consist of a few coupled quantum wells; see, generally, Sze, Physics of Semiconductor Devices, 729-730 (Wiley Interscience, 2d Ed 1981). High Electron Mobility Transistors (HEMTs) are another type of quantum well device and typically use only one half of a quantum well (a single heterojunction) but may include a stack of a few quantum wells. The HEMT properties arise from conduction parallel to the heterojunctions and in the quantum well conduction or valence subbands; the conduction carriers (electrons or holes) are isolated from their donors and this isolation limits impurity scattering of the carriers. See, for example, T. Drummond et al, Electron Mobility in Single and Multiple Period Modulation-Doped (Al,Ga)As/GaAs Heterostructures, 53 J.Appl.Phys.1023 (1982). Superlattices consist of many quantum wells so tightly coupled that the individual wells are not distinguishable, but rather the wells become analogous to atoms in a lattice. Consequently, superlattices behave more like new types of materials than as groups of coupled quantum wells; see, generally, L. Esaki et al, Superfine Structure of Semiconductors Grown by Molecular-Beam Epitaxy, CRC Critical Reviws in Solid State Sciences 195 (April 1976). Superlattices have been used as quantum well barriers to improve the well-barrier interface by lessening trapping of undesirable impurities and preventing surface roughening during growth; see the photoluminescence studies of H. Sakaki et al, Energy Levels and Electron Wave Functions in Semiconductor Quantum Wells Having Superlattice Alloy like Material (0.9 nm GaAs/0.9 nm AlGaAs) as Barrier Layers, 47 Appl.Phys.Lett. 295 (1985) and K. Fujiwara et al, 24 Jpn.J.Appl.Phys. Part 2 L405 (1985).
Resonant tunneling devices are the simplest quantum well devices that exhibit quantum confinement and coupling and were first investigated by L. Chang et al, 24 Appl.Phys.Lett. 593 (1974), who observed weak structure in the current-voltage characteristics of resonant tunneling diodes at low temperatures. More recently, Sollner et al, 43 Appl.Phys.Lett. 588 (1983), have observed large negative differential resistance in such devices (peak-to-valley rats as large as six to one have been obtained), and Shewchuk et al, 46 Appl.Phys.Lett. 408 (1985) and M. Reed et al, J.Mat.Res. (1986), have demonstrated room temperature resonant tunneling.
A typical resonant tunneling diode structure is schematically illustrated in FIGS. 1A-D; FIG. 1A is a schematic cross sectional view, FIG. 1B illustrates the profile of the conduction band edge through such a diode with no bias, FIG. 1C is the conduction band edge for bias into resonance, and FIG. 1D is a typical current-voltage characteristic for the diode at low temperature. The preferred material is the lattice matched system of GaAs/Al.sub.x Ga.sub.1-x As, although resonant tunneling has been observed in strained-layer heterostructure systems; see Gavrilovic et al, 52 Solid State Comm. 237 (1984). The two Al.sub.x Ga.sub.1-x As layers that define the central GaAs quantum well (see FIGS. 1B-C) serve as partially transparent barriers to electron transport through the diode. Resonant tunneling occurs when the bias across the outer terminals is such that one of the quantum well bound states has approximately the same energy level as the input electrode Fermi level. This is illustrated by the arrows in FIG. 1C. Peaks in the electron transmission (current) as a function of bias (voltage) are thus observed. The resonant tunneling diode is the electrical analog of a Fabry-Perot resonantor. Leakage (inelastic tunneling current) is determined by the quality of the GaAs/Al.sub.x Ga.sub.1-x As interfaces and electron-phonon scattering.
The resonant tunneling diode has high speed charge transport (less than 100 femtoseconds) which implies applications to microwave oscillators and high speed switches. But the utility of such diodes is limited by inelastic tunneling and scattering in the tunneling barriers which degrade performance.
Also, quantum well devices are typically grown layer by layer with molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD), and the substrate temperature during such growth leads to the diffusion of doping impurities into the undoped tunneling barriers and well and consequent performance decline. Further, MBE and MCVD have difficulty with control of the alloy composition (the x) in Al.sub.x Ga.sub.1-x As growth, which is another source of performance degradation.
Thus known resonant tunneling diodes have the problems of interface imperfections and alloy scattering by alloy barriers, and fabrication problems of alloy composition control and doping impurity diffusion from the heavily doped electrodes through the barriers.